"Riding your Computer Up" vs. "Lite Deco"

[scubadada]

How about it? If you're solo diving you should already have a redundant gas supply. Make sure it's got adequate capacity to honor your obligation, should one occur. It ain't rocket surgery ...

... Bob (Grateful Diver)

correct

 

[KenGordon]

I ran the Buhlmann profiles on MultiDeco. The BSAC 88 NDLs look perfectly reasonable, it's the deco that surprised me. If you do any deco using the tables, you end up with surfacing code G. It takes quite a long SI to reduce that to a code where you can really dive again. I would imagine most Sports Divers dive a computer with a standard decompression algorithm these days.

This illustrates that NDL limits do not completely map to aggressiveness.

If you plan your dive with these tables and dive them with a computer the various roundings and the difference between planned and actual depth, descent rate etc tend to cancel out. So as planning tools they are vaguely ok, definitely ok for no stop dives.

They were introduced just as dive computers started to make an impact so it might not be fair to assume they have been very heavily tested by people using them.

I'd use them for an emergancy ascent, but not for a week of lots of deep and long dives.

I do know at least one diver who does dive them (and the ox-stop accelerated tables) though. He is still walking.

There is more to a set of tables than the first dive. Many issues involve repeat diving, so you have to consider how aggressive all these scenarios are.

 

[DevonDiver]

BSAC tables are notoriously aggressive [...] I don't know anyone who actually dives the 88s.

Decompression preferences have certainly evolved a long way since 1988.

When I first started technical diving ('88 tables were already pretty old even then), we were planning very Haldanian ascents on MS-DOS planners. There were divers who felt it normal or acceptable to feel "a little fizz in the joints" post-dive.

If you do any deco using the tables, [...] It takes quite a long SI to reduce that to a code where you can really dive again.

As to be expected from a navy derived table...

My personal opinion is that antiquated approaches should be consigned to the museum. Compared to modern algorithms, the '88 tables are antiquated.

I wouldn't even compare them to Buhlmann ZH-L16 or VPM-B/E. Even the most basic modern entry-level 'recreational' dive computers produce more refined ascents.

Of course, '88 tables are proven safe by decades of application...but 'safe' is a relative term. Most modern algorithms are 'safer' (if we assume longer deco and a more curved, less linear/angular profile promotes cleaner surfacing).

Safe is ok. Safer is better.

The trend is towards more conservative deco and less acceptance of risk.

I don't want my technical diving to be safe... I want it to be as absolutely safe as possible. I don't mind hanging longer, for the peace of mind it brings. I've never been bent... I never intend to get bent.

 

[Storker]

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A ScubaBoard Staff Message...

The ensuing discussion about BSAC-style club diving versus commercial diving operators has been moved to its own thread.

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[lowviz]

...//... When I first started technical diving ('88 tables were already pretty old even then), we were planning very Haldanian ascents on MS-DOS planners. There were divers who felt it normal or acceptable to feel "a little fizz in the joints" post-dive.



As to be expected from a navy derived table...

So using the US Navy tables would be a reasonable "absolute end of the line" NDL ascent? That is how I see it, go no further unless you are doing real deco totally by the books.

But let's compare the PADI RDP to navy tables. The RDP is very conservative down to about 100' and then becomes much more aggressive to the point that it differs from the navy table by only a minute at 130' on air. The navy tables are funny, too. Five minutes for anything past 150'? So I used the shallower parts of the US Navy table to predict deeper NDL's. That is where the 222222/(depth squared) stuff came from. It was nice to see that VPM-B with no conservatism and about 4 min required deco matches my approximation rather well. See pic:

The understanding gained from all this is that the RDP is conservative while shallow and rather aggressive at depth. So one can't get away with expecting that PADI is evenly conservative across the board. I can fudge it a bit when shallow and get away with it, but not at depth.

...//... My personal opinion is that antiquated approaches should be consigned to the museum. Compared to modern algorithms, the '88 tables are antiquated. ...

They were developed under unpublished assumptions/observations/calculations if I am correct. As such, I am very interested in comparing them to what I now have for new insights.

...//... I wouldn't even compare them to Buhlmann ZH-L16 or VPM-B/E. Even the most basic modern entry-level 'recreational' dive computers produce more refined ascents. ...

No doubt. But I'm planning to compare them to exactly that. I was most kindly offered a link to BSAC where non-members can purchase supplies. Did so, now waiting for my BSAC tables to make their way through the holiday mail.

...//...Of course, '88 tables are proven safe by decades of application...but 'safe' is a relative term. ...

I can't wait to compare '88 tables to navy tables and then do an approximation. The difference between the two approximations will be telling either way.

...//... The trend is towards more conservative deco and less acceptance of risk. ...

Right there with you on that thought.

But knowing a bit about the fuzzy gray area works better for me than just watching in horror as the NDL line approaches.

 

[boulderjohn]

You cannot compare the PADI and Navy tables by simply looking at the first dive NDLs. There is a reason for what you are seeing there, and that reason plays out in a repetitive dive schedule.

The Navy tables were designed for people doing one dive a day, and those dives generally looked different from the ones recreational divers do. They based their repetitive dive system on the 120 minute compartment, which results in extremely long surface intervals between dives. This was a real problem for people doing multiple dive days. PADI's research indicated that for the kinds of dives normally done by recreational divers, the kind profiled in the RDP, the 40 minute compartment was suitable for surface interval planning. To make it more conservative, they used the 60 minute compartment. This greatly reduced required surface intervals. They then shortened the NDLs for most first dives, which again shortened surface intervals. Finally, they greatly increased the number of pressure groups, which lessened the amount of rounding to be done and further shortened surface intervals.

As a consequence, the HUGE difference in between the PADI tables and the Navy tables is not the first dive NDLs, it is the amount of time required between dives. The PADI tables essentially allowed for the concept of the standard recreational 2-tank dive, which could not really be done with the Navy tables.

 

[lowviz]

Let me preface this post with my respect for your past experience and ability to "popularize" (explain in words) the same.

You cannot compare the PADI and Navy tables by simply looking at the first dive NDLs. There is a reason for what you are seeing there, and that reason plays out in a repetitive dive schedule. ...

Yes, I can compare them for a single tank dive.

I need to start with the fundamentals and build on that. We will get to second dives later. It is my presumption that the navy neither wants to molly coddle nor bend its resources. Its resources are men and women in their prime. One working dive a day, then give the kids a break. So, I remain convinced that their stats tell them (the navy) that we can push them (the kids) this hard and then back off. Five minutes for anything over 150'? How about "You, go down there and get that (whatever) that the last diver dropped." Nothing more than a justified bounce dive for a specific purpose. The navy isn't stupid, even though that is a popular concept for non-military minds. The Hackensack: Using Bayesian Analysis to find a lost nuclear weapon

So, I take the US Navy tables to be my "Go no Further" signpost. Anything past that is real deco and done with religion and by the tables. Jesus (or yours) take the wheel, beyond my power to compute...

...//... The Navy tables were designed for people doing one dive a day, and those dives generally looked different from the ones recreational divers do. They based their repetitive dive system on the 120 minute compartment, which results in extremely long surface intervals between dives. This was a real problem for people doing multiple dive days. PADI's research indicated that for the kinds of dives normally done by recreational divers, the kind profiled in the RDP, the 40 minute compartment was suitable for surface interval planning. To make it more conservative, they used the 60 minute compartment. This greatly reduced required surface intervals. ...

I love this equivocation. OK, so we can't do two tank dives on navy tables. How about we call BS on their leading compartment? Let's use a shorter one, that works.

Second dive. Who is more aggressive, USN or sport?

...//... As a consequence, the HUGE difference in between the PADI tables and the Navy tables is not the first dive NDLs, it is the amount of time required between dives. The PADI tables essentially allowed for the concept of the standard recreational 2-tank dive, which could not really be done with the Navy tables.

I see both sides of this issue.

So PADI cloaks themselves in being far more conservative, yet the USN wouldn't touch that profile?

 

[dumpsterDiver]

I remember when the PADI tables came out. We had been following the NAVY tables.

Another way of looking at it (even simpler than Boulder John's explanation, but not contradicting it in any way) is that all of a sudden.. if we did a shorter dive for our first dive, we could take a shorter surface interval (around 45 minutes gets you a good bit of time back) and dive longer for the second dive .. with the resulting situation being that we could do more TOTAL diving in 2 dives than we could before. Pretty crazy, really.

I look back on some of the stuff we did in the 70's in NJ. A couple dives to 80 to 100 mostly and then if you were good enough, you got to (after a 10 minutes surface interval) bounce down to the bottom (solo) with several hundred psi in the tank, while the boat idled forward, unhook the grappling hook and chain, give three big tugs on the rope, and the mate would haul the anchor and you (as you held onto the hook) back to the surface as fast as he possibly could haul it in. It was a fun ride getting hauled to the surface like that. We didn't need any stops or anything because it was "just a bounce dive"..

 

[shoredivr]

Over the years, past threads have pointed out that in a typical scuba course, we spend a lot of time dealing with the dangers of decompression sickness. As I mentioned earlier, the way we taught and used tables emphasized the danger of exceeding NDLs for even one minute. [....]

I briefly mentioned the dive buddy I had who made a panicked ascent when he saw his computer was getting near deco. I wonder how many fatalities were caused when people saw they had reached their NDLs and had to get to the surface in a hurry to prevent the horror of straying into deco.

Not many I'd think. DCS is a very intangible, non-instinctive risk psychologically. There's no direct 'trigger' to our fight or flight instincts. Very unlike, for instance, having no breathing air, flooding a respiratory passage with water, or even seeing predatory marine life.

A decision to ascend quicker than optimally due to numbers on a computer screen is an error in comparative risk analysis. It's an intellectual process, albeit a flawed one, not an innate self-preservative reaction.

I agree with Boulderjohn, though. When I'd finished OW and AOW (PADI) here in Canada in 1998, I had *no idea* about staged decompression. The facts drilled into me were that DCS can maim or kill you. And that to go over NDL, by even a minute, was, therefore, potentially going to maim or kill me. Because I had not seen a decompression table, I had no idea one could spend x minutes at y depth during decompression and so on. (Of course, I had no idea how to calculate gas consumption, or hold a stop in the water column for x minutes without drifting up or down either, some of the things you need to have a firm grasp on to do decompression diving).

Because I'm detail oriented, I used to get anxious as NDL approached, and a number of times went up ten or twenty feet above buddies who were not so observant of their computers.
No so good for buddy protocol, but I didn't want to get bent. My then incomplete knowledge of how deco diving was performed led me to think only of maiming and death as those numbers got closer to zero on my computer. On top of this, I had no idea what my computer looked like if it went into deco, and no idea of what to do if it did, or how to interpret what it said. It was just a black hole of doom to be avoided at all costs.

Over the years I have seen divers bolt to the surface as they approached deco. They had no idea of what to do either as those NDL numbers got to 3, 2, 1....other than go up quickly.

And that can become an uncontrolled ascent. Especially in a drysuit. It didn't matter that responding to the numbers by quickly ascending was a "flawed intellectual process", they still went up dangerously quickly. Think embolism potential.

*****

When I took decompression training, in my first actual planned deco dive, as I watched my computer tick over 0 NDL and I read the word DECO, *even though I was in decompression training*, I still had that little voice from OW/AOW saying "you're gonna die, go up now". Of course, I did not.

*****

By providing little
to no practical education in OW/AOW about what to do when you (accidentally or on purpose) go really close to or over NDLs, there is definitely a subset of people who will make a panicky choice to go up quickly if that occurs, with the potential of an uncontrolled ascent and the dangers of embolism.

This leads me to another point about training. The blanket theoretical proclamation on an OW/AOW table of stay 8 minutes at 15 feet (or whatever) if you exceed NDL doesn't cut it if these skills have never been practiced.

Do you know how much gas you need to hang out there at 15 ft? Are you properly weighted for that 15 ft stop; realizing that as you use up gas normally, you lose the weight of the gas and become more buoyant?

If you've just got stressed as you approach or exceed NDLs you breathe faster. Rapid, stressed breathing holds more gas in the lungs, and uses more gas, both of which increase buoyancy, making that 15 ft stop even harder to hold. That in turn causes stress as you struggle to hold your depth (if not properly weighted), in a cycle that can pop you up to the surface in no time.

How can you hold that stop for 8 minutes, without going above 15 ft with, say 800psi, if you've never practiced in a *benign* situation? OW/AOW training should encompass this benign scenario IMO, instead of it being a theoretical thing on a table, to do if you blow that NDL.

 

[lermontov]

I remember when the PADI tables came out. We had been following the NAVY tables.

Another way of looking at it (even simpler than Boulder John's explanation, but not contradicting it in any way) is that all of a sudden.. if we did a shorter dive for our first dive, we could take a shorter surface interval (around 45 minutes gets you a good bit of time back) and dive longer for the second dive .. with the resulting situation being that we could do more TOTAL diving in 2 dives than we could before. Pretty crazy, really.

I look back on some of the stuff we did in the 70's in NJ. A couple dives to 80 to 100 mostly and then if you were good enough, you got to (after a 10 minutes surface interval) bounce down to the bottom (solo) with several hundred psi in the tank, while the boat idled forward, unhook the grappling hook and chain, give three big tugs on the rope, and the mate would haul the anchor and you (as you held onto the hook) back to the surface as fast as he possibly could haul it in. It was a fun ride getting hauled to the surface like that. We didn't need any stops or anything because it was "just a bounce dive"..

real diving like lloyd bridges from sea hunt!!!